1. Field of the Invention
The present invention relates to a common-mode feedback circuit and a differential operational amplifier circuit, and in particular to a common-mode feedback circuit that outputs a control voltage for defining the common-mode operating point of a fully differential amplifier.
2. Description of the Related Art
In the prior art, there is proposed a common-mode feedback circuit that outputs a control voltage for defining the common-mode operating point of a fully differential amplifier. That is, a fully differential amplifier is a circuit that amplifies differential input voltages and outputs the amplified signals as differential output voltages, and this amplifier requires the provision of a common-mode feedback circuit for controlling the common-mode output voltage. The common-mode feedback circuit receives the non-inverting output (positive logic output) and inverting output (negative logic output) of the fully differential amplifier as input voltages, and generates a control voltage for controlling the common-mode operating point of the fully differential amplifier.
In the prior art, it is known to provide a common-mode feedback circuit in which the outputs of a fully differential amplifier are input via a voltage dividing circuit to a differential amplifier whose output is fed back to the fully differential amplifier (for example, with reference to Japanese Unexamined Patent Publication (Kokai) No. 61-109309 (FIG. 5)). In the prior art, there is also proposed a common-mode feedback circuit (differential operational amplifier circuit) in which the outputs (differential outputs) of a fully differential amplifier are received by two buffer circuits whose outputs are divided through a voltage dividing circuit to obtain a midpoint voltage (for example, with reference to Japanese Patent No. 2884896 corresponding to Japanese Unexamined Patent Publication (Kokai) No. 05-283952 (FIGS. 1 and 3)).
There is also proposed in the prior art a common-mode feedback circuit that holds the common-mode signal of the fully differential amplifier fixed accurately at a desired potential and can obtain a sufficiently high common-mode feedback gain (for example, with reference to Japanese Unexamined Patent Publication (Kokai) No. 06-232654). In another common-mode feedback circuit proposed in the prior art, stability is achieved without increasing load capacitance (for example, with reference to Japanese Unexamined Patent Publication (Kokai) No. 07-007340). In yet another common-mode feedback circuit proposed in the prior art, the circuit configuration is simplified by reducing the number of devices (for example, with reference to Japanese Unexamined Patent Publication (Kokai) No. 07-273567).
The prior art and its associated problems will be described later with reference to the accompanying drawings.